A pathogen that has spread through forests in the West has a deadly side effect: It makes redwood trees more likely to die in fires.

The pathogen, Phytophthora ramorum, causes sudden oak death (SOD). Oaks throughout California and Oregon have fallen prey to the disease.

The study authors began monitoring trees hit by SOD over 80,000 hectares of Big Sur, California in 2006. The researchers’ 280 plots included tanoaks, California bay laurels, and coast redwoods. In 2008, nearly half the plots were scorched by wildfires that lasted more than a month.

In the aftermath of the fire, the team returned to 46 plots. Half the sites had been infested with the pathogen, and half had not. Even though redwoods are known for being fire-resistant, the redwoods’ mortality risk in the infested plots was as much as four times higher than in the healthy plots, the researchers report in Ecology.

The disease may have created more fuel for fires because dead tanoak branches dropped to the ground, the authors speculate. And the loss of trees would have reduced the amount of shade, drying out the wood. Together, these changes to the forest “overwhelmed redwood’s usual resilience to wildfire,” the team writes. — Roberta Kwok | 25 July 2013